A fuel cell generates electricity by an electrochemical reaction between a fuel and an oxidizer. The fuel cell includes a membrane electrode assembly (MEA) in which a fuel electrode and an oxidizer electrode are arranged opposite each other on respective sides of an electrolyte formed of an ion exchange membrane; a fuel separator in which a fuel supply channel is formed to supply the fuel to the fuel electrode; and an oxidizer separator in which an oxidizer supply channel is formed to supply the oxidizer to the oxidizer electrode. Various gasses are used for the fuel and the oxidizer. For example, hydrogen is used as the fuel, and air containing oxygen is used as the oxidizer in many types of fuel cells. Electricity is generated by an electrochemical reaction therebetween, and in many types of fuel cells water is often generated on the oxidizer electrode.
As shown in, for example, Japanese Patent Publication Laid-open No. 2006-221836, when the operation of such a fuel cell is stopped, air, which serves as an oxidizer gas, remains in the oxidizer supply channel on the side of the oxidizer electrode, and hydrogen, which serves as a fuel gas, remains in the fuel supply channel on the side of the fuel electrode. Meanwhile, in the fuel cell being stopped, cross leakage occurs, wherein hydrogen which serves as the fuel gas moves to the oxidizer electrode through the ion exchange membrane, while oxygen in the air which serves as the oxidizer gas moves to the fuel electrode through the ion exchange membrane. When this cross leakage occurs, hydrogen is bonded to oxygen by a chemical reaction different from a power-generating reaction, so that water is generated.
The reaction between hydrogen and oxygen by the cross leakage stops when oxygen in the air is used up. However, if new air flows into the oxidizer supply channel when the fuel cell is not in operation, the above-mentioned reaction by the cross leakage occurs continuously. Then, catalysts contained in the fuel electrode and the oxidizer electrode deteriorate due to an increase in the potentials of the oxidizer electrode and the fuel electrode in the fuel cell, so that a catalytic performance deteriorates, which leads to a problem of performance deterioration of the fuel cell.
In order to prevent such a performance deterioration of the fuel cell, Japanese Patent Publication Laid-open No. 2006-221836 describes a method wherein normally-closed electromagnetic valves which are closed by the cutoff of a driving power source are provided in conduits at the oxidizer gas inlet and outlet of the fuel cell, whereby the oxidizer electrode is easily sealed even when the fuel cell is not in operation. Furthermore, Japanese Patent Publication Laid-open No. 2000-3717 describes a system which drives a shutoff valve of a fuel cell by air pressure.
Meanwhile, as a shutoff valve for the fuel cell used in some cases, there is a valve type (hereinafter referred to as an upstream opening type) in which a movable part of the valve moves from the downstream side to the upstream side of an oxidizer gas to open the valve. In the valve having such a configuration, if the oxidizer gas flows into a gas passage in the valve, the movable part of the valve is pushed in a valve-closing direction (from the upstream side to downstream side of the flow of the oxidizer gas) by the pressure of the gas, as disclosed in, for example, Japanese Patent Publication Laid-open No. 2006-24469.
The shutoff valves provided at the oxidizer gas inlet and outlet of the fuel cell need to always maintain an open state when the fuel cell is in operation. Thus, in order to ensure the open state of the valves when the fuel cell is in operation, a normally-open valve may be used instead of the normally-closed valve as described in Japanese Patent Publication Laid-open No. 2006-221836. The normally-open valve maintains the open state when a driving source such as a valve opening/closing power source or air source is stopped. In such a normally-open valve, the driving source such as the power source or air source needs to be provided to keep the shutoff valve close when the fuel cell is not in operation. Therefore, when the driving source is stopped, the shutoff valve may open, or closing force may decrease, so that air may leak in.
Furthermore, as the conventional technique described in Japanese Patent Publication Laid-open No. 2000-3717, when the shutoff valve is driven by air pressure, the pressure of the pressurizing air in a pressurizing chamber decreases due to, for example, leakage of air. Thus, the shutoff valve may open, or closing force may decrease, so that air may leak in.
Meanwhile, since the oxidizer gas in the fuel cell contains moisture, the shutoff valve for the oxidizer gas may be firmly fixed or frozen in a closed state if the fuel cell is stopped in a low-temperature environment. When the shutoff valve is an upstream-opening-type valve as in the conventional technique described in Japanese Patent Publication Laid-open No. 2006-24469, the pressure of the oxidizer gas supplied for starting operation acts in a valve-closing direction to hamper the opening of the valve. Consequently, great driving force may be required.
An object of the present invention is to improve sealing properties of a shutoff valve during stoppage of a fuel cell and to improve opening properties of the shutoff valve when the fuel cell is started.